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NOTE: Originally written by Prof. J. McNeill Modified 2/16/05 by S. J. Bitar ECE 2201 LAB 5 - BJT Fundamentals PRELAB P1. Meter The circuit of Figure P5-1 can be used to measure the current gain of the BJT. Determine values for resistors R1 and R2 to meet the following conditions: The base current IB ≈ 10µA (choose R1 as the closest standard value from your lab kit) The voltage drop across R2, expressed in mV, gives the BJT current gain . That is, VDVM (1mV) Note that R2 will not be a standard value; we'll worry about that in lab... + +10V R2 R1 IC C IB Q1 2N3904 Figure P5-1. 1 E + VDVM - EE2201 - LAB 5 BJT Fundamentals and Applications: iC-vBE Characteristic Analog Amplifier Applications PURPOSE: The purpose of this laboratory assignment is to investigate the NPN bipolar junction transistor (BJT). Upon completion of this lab you should be able to: Measure the current gain of the BJT. Extract BJT parameter IS from vBE-iC measurements. Apply the BJT in a linear amplifier application in moderate gain configuration (common emitter with emitter degeneration resistor RE) Use a large-value capacitor to short the emitter to ground at signal frequencies to use the BJT in a linear amplifier application with a high gain configuration (common emitter) Use a large-value capacitor to couple an AC signal to the input of the common emitter amplifier MATERIALS: ECE Lab Kit DC Power Supply DVM Function Generator Oscilloscope NOTE: Be sure to record ALL results in your laboratory notebook. 2N3904 BJT terminal designation reminder: COLLECTOR BASE EMITTER (TOP VIEW) 2 BIPOLAR JUNCTION TRANSISTOR (BJT): iC-vBE CHARACTERISTIC L1. Build the BJT circuit shown in Fig. 5-1, using the 2N3904 NPN BJT. By using different values for resistors RB and RC, you will measure the base current iB, collector current iC, and base-emitter voltage vBE over a range of DC collector currents. Note: Be sure to measure the actual voltage of the +10V power supply for VCC. Note: Capacitors CBP1 and CBP2 are open at DC, and don’t affect your measurements of DC current or voltage. Their purpose is to filter out high frequency noise that can be coupled into this (high gain) circuit when measurements are being made. VCC = +10V + +10V IR CBP1 0.1µF RC IL RB IC IB B + VBE - CBP2 0.1µF + VRC - C Q1 2N3904 E Figure 5-1. L2. Vary RB and RC using the range of values shown in the table below. For each set of values, measure VBE and VRC. Calculate IB and IC , and using: (1) IC=VRC/RC (2) IB=(VCC-VBE)/RB (3) = IC/IB Note that Eq. (2) assumes IL is negligible, so that IC≈IR. Nominal RB Actual (Measured) RC 43 kΩ * 25 Ω 100 kΩ 51 Ω 200 kΩ 100 Ω 430 kΩ 200 Ω RB RC Measured VRC 1 MΩ 510 Ω * for 25Ω, use two 51Ω in parallel. 3 VBE Calculated IC IB Meter L3. Modify the circuit of Figure 5-1 by changing the values of RB and RC to implement your meter design from the prelab. TRIMMING For the resistor value that is not standard, you will need to trim to an exact value by using series and parallel combination of resistors in your kit. Since you know from your results in part L2 what the correct value of is, you can start calibration of your meter circuit by making the collector resistor a few percent higher than the “exact” value. This will give a reading on the DVM higher than the correct . Then you can adjust (trim) by adding resistance in parallel to reduce the total collector resistance until the correct is displayed. Trimming hint: it can be shown that (for large x), if a resistor of value R needs to be reduced to a value (1 - 1/x)R, this can be achieved by adding a resistor of value xR in parallel. For example, to reduce a resistor value R by 5%, add a resistor of value 20R in parallel. You can do the same thing with a series trim, but inserting resistors in series requires you to break connections – not easy to do on a printed circuit board in a production environment! An additional advantage to trimming from the output is that you will also take into account the effect of tolerance errors on the value of RB, as well as variations in VBE away from the 0.7V value assumed in the prelab. L4. Compare the performance of your trimmed circuit to the measurements from part L2. How do the resistor values you used in lab compare to the design values from prelab? 4 COMMON EMITTER AMPLIFIER WITH DEGENERATION RESISTOR RE L5. Build the amplifier circuit shown in Fig. 5-2. Note that the supply voltage is increased to +15V (measure the exact value to use in IC calculations). VCC = +15V RC 20kΩ + +15V VOUT C BP1 0.1µF IC Vin (0.1V)sin2 (10kHz)t + + VB E - - + Q1 2N3904 VE RE 1k Ω +1.0V FUNCTION GENERATOR Figure 5-2 L6. For the input signal source (a 10kHz, 0.1V peak sine wave riding on a 1.0V DC level) use the function generator with the DC offset enabled (pull out the DC OFFSET knob). Display vIN on oscilloscope channel 1; set the horizontal time scale to show a few cycles of the sine wave. To set the DC offset level, set the scope to DC coupling; view the signal at 0.5V/div, and adjust the function generator offset until the DC level of the signal is 1.0V. To set the peak amplitude, go to AC coupling on the scope and “zoom in” to a vertical resolution of 50mV/div, and adjust the function generator amplitude until the peak level of the signal is 0.1V (peak-to-peak level of 200mV). DC BIAS LEVEL L7. Display vOUT on oscilloscope channel 2. Set both channels to 1V/div and adjust the vertical position so that 0V (ground) is at the bottom of the screen. L8. Measure the average DC level of the input and output voltages (either using the DVM or the MEAN function of the scope MEASURE function). Determine the DC operating point (bias current) IC for the BJT. 5 BASE-EMITTER JUNCTION OPERATION (0.7V VBE) L9. Change scope channel 2 to show the emitter voltage VE. You should see it “follow” the base voltage vIN, offset by the 0.7V drop of the base-emitter junction. We’ll take advantage of this property in the “emitter follower” configuration, a unity-voltage-gain buffer that provides current gain so that a high-impedance source can drive a low-impedance load. SMALL SIGNAL GAIN L9. Change scope channel 2 back to show vOUT. Verify the 180° phase shift (inversion of the sine wave) from input to output. L10. Measure the input and output signal peak-to-peak signal amplitudes vin(p-p) and vout(p-p). To get an accurate measurement, you may want to go to AC coupling and “zoom in” to a finer vertical scale. Note the shape of the output waveform – is it a clean sinusoid, or is there distortion? Measurement note: The peak-to-peak measurement function will give a slightly larger value than the true peak-to-peak amplitude, due to noise (“fuzz”) on the waveforms. Instead, use the cursors and eyeball the cursor locations to the “middle” of the fuzz. L11. Determine the measured small-signal voltage gain av (calculated as av = vout(p-p)/vin(p-p). Compare to the theoretically predicted value RC/RE. USE OF CAPACITOR TO SHORT EMITTER TO SIGNAL GROUND L12. Go back to DC coupling of both channels for vIN and vOUT. Set both channels to 2V/div and adjust the vertical position on each so that 0V (ground) is at the bottom of the screen. VCC = +15V RC 20kΩ + +15V VOUT C BP1 0.1µF IC Vin (0.1V)sin2 (10kHz)t + + VB E - - + Q1 2N3904 VE RE 1k Ω +1.0V FUNCTION GENERATOR + ADD Figure 5-3 6 CE 100µF L13. Add a 100µF capacitor in parallel with RE as shown in Figure 5-3. Be sure to observe correct polarity! You should see the output waveform amplitude increase significantly, due to the increase in signal gain. The output amplitude should exceed the linear range of the amplifier, causing the output “sine wave” to be severely distorted (“clipped”). Measure the maximum and minimum voltage levels at the output, and (in your writeup) identify the transistor operating regions corresponding to each limit. AMPLIFIER APPLICATION: COMMON EMITTER L14. Build the amplifier circuit shown in Fig. 5-4. VCC = +15V RB2 43k Ω + +15V CBP 0.1µF 1 RC 20k Ω VOUT IC CB 100µF RS1 1k Ω Vs + (0.5V)sin2(10kHz)t - Vin Q1 2N3904 + RS2 10 Ω FUNCTION GENERATOR VE RB1 3k Ω RE 1k Ω + CE 100µF Figure 5-4 INPUT ATTENUATOR L15. Since the gain of this amplifier is over 100, it is necessary to make a very small input voltage (a few millivolts peak-to-peak) to avoid clipping at the output. Resistors RS1 and RS2 form a 100:1 attenuator, so a 0.5V peak sine wave at the function generator produces a 5mV peak sine wave at Vin. Since the 10mV peak-to-peak signal amplitude at the amplifier input Vin may be too small to measure directly on the oscilloscope, you’ll measure it indirectly by measuring the amplitude of Vs at the function generator output, and dividing by the attenuation ratio of the voltage divider network (measure RS1 and RS2 to get the correct actual values for the voltage divider expression). 7 DC BIAS NETWORK L16. Resistors RB1 and RB2 form a voltage divider network that sets the DC bias level (operating point) at VIN equal to approximately 1.0V (the same as the amplifier circuit from the previous section). The small signal input is coupled to Vin using capacitor CB (be sure to observe correct polarity!). Since the capacitor is open at DC, coupling the input signal in this way allows the designer to treat the DC bias and AC signal amplification problems separately, thus simplifying the design process. DC BIAS LEVEL L17. Display vIN and vOUT on oscilloscope channels 1 and 2. Set both channels to 1V/div and adjust the vertical position so that 0V (ground) is at the bottom of the screen. L18. Measure the average DC level of the input and output voltages (either using the DVM or the MEAN function of the scope MEASURE function). Determine the DC operating point (bias current) IC for the BJT. EMITTER “SHORTED” TO SIGNAL GROUND BY CE L19. Change scope channel 1 to show the emitter voltage VE. Even as the output voltage varies significantly, you should see a constant voltage (no signal activity) at VE. This indicates that the presence of capacitor CE has “shorted” the emitter voltage to signal ground. SMALL SIGNAL GAIN L20. Change scope channel 1 to show the function generator output vS. Verify the 180° phase shift (inversion of the sine wave) from input to output. L21. Measure the input and output signal peak-to-peak signal amplitudes vin(p-p) and vout(p-p). To get an accurate measurement, you may want to go to AC coupling and “zoom in” to a finer vertical scale. Note the shape of the output waveform – is it a clean sinusoid, or is there distortion? Measurement note: you will have to determine the input amplitude vin(p-p) indirectly as described in L15. L22. Determine the measured small-signal voltage gain av (calculated as av = vout(p-p)/vin(p-p). Compare to the theoretically predicted value RC/re. 8 EXCEEDING “SMALL SIGNAL” LINEAR RANGE L23. Go back to DC coupling of both channels for vIN and vOUT. Set both channels to 2V/div and adjust the vertical position on each so that 0V (ground) is at the bottom of the screen. L24. Change the input waveform from a sine wave to a triangle wave. This should allow you to see nonlinear distortion more clearly. Increase the input amplitude and note how the nonlinearity (distortion) of the output waveform gets worse as amplitude increases. Even before the severe distortion of clipping, you should see significant nonlinearity for large output waveforms. In your writeup, explain this behavior in light of the small-signal linear approximation. L25. Increase the input amplitude until the output exceeds the linear range of the amplifier, causing clipping. Measure the maximum and minimum voltage levels at the output, and (in your writeup) identify the transistor operating regions corresponding to each limit. NOTE: If possible, save yourself some time next week and DO NOT DISASSEMBLE THIS CIRCUIT! WE WILL BE MEASURING ITS LIMITATIONS IN THE FREQUENCY DOMAIN (BANDWIDTH) IN LAB 6 9 LAB WRITEUP W1. Plot IC as a function of vBE on a semilog scale. Determine the scale current parameter IS (assume n=1), and plot the prediction of the IC=ISeVBE/Vt model on the same axes as your data. How well does the model agree with your data? W2. Plot your calculated as a function of IC. How constant is the “constant” ? Over the current range for which you took measured data, what (constant) value for would you use, and how much error would you make in assuming constant? Meter W3. Compare the performance of your trimmed circuit to the measurements from part L2. How do the resistor values you used in lab compare to the design values from prelab? COMMON EMITTER WITH EMITTER DEGENERATION RESISTOR RE W4. Analysis - From a large signal analysis of the circuit in Figure 5-2, determine the theoretically predicted DC operating point for collector current and the average DC level of the input and output voltages. W5. Compare your measured results from L8 to the theoretical prediction of W4. BASE-EMITTER JUNCTION OPERATION (0.7V VBE) W6. Plot (as a function of time) the base voltage vIN and the emitter voltage VE from your oscilloscope observation in lab part L8. Did you observe VE “following” the base voltage vIN, offset by the 0.7V drop of the base-emitter junction?. SMALL SIGNAL GAIN W7. Plot vIN and vOUT (as a function of time) from your oscilloscope observation in lab part L9. Verify the 180° phase shift (inversion of the sine wave) from input to output. W9. Analysis - From a small signal analysis of the circuit in Figure 5-2, determine the theoretically predicted output peak-to-peak signal amplitude vout(p-p) (given your measured input peak-to-peak signal amplitude vin(p-p)) and the small signal gain av. W10. Compare your measured results for output peak-to-peak signal amplitudes vout(p-p) and small-signal voltage gain av Compare to your theoretically predicted values from W9. W11. Comment on the shape of the output waveform – was it a clean sinusoid, or was there distortion? 10 USE OF CAPACITOR TO SHORT EMITTER TO SIGNAL GROUND W12. When the 10µF capacitor was added in parallel with RE (as shown in Figure 5-3), how did circuit operation change? Describe qualitatively and quantitatively. W13. Plot the output waveform, identifying the measured maximum and minimum voltage clipping levels. Identify the transistor operating regions corresponding to each limit. AMPLIFIER APPLICATION: COMMON EMITTER DC BIAS LEVEL W14. Analysis - From a large signal analysis of the circuit in Figure 5-4, determine the theoretically predicted DC operating point for collector current and the average DC level of the input and output voltages. W15. Compare your measured results from L18 to the theoretical prediction of W14. SMALL SIGNAL GAIN W16. Plot vIN and vOUT (as a function of time) from your oscilloscope observation in lab part L20. Verify the 180° phase shift (inversion of the sine wave) from input to output. W17. Analysis - From a small signal analysis of the circuit in Figure 5-4, determine the theoretically predicted output peak-to-peak signal amplitude vout(p-p) (given your measured input peak-to-peak signal amplitude vin(p-p)) and the small signal gain av. Remember that you will have to determine the input amplitude vin(p-p) indirectly as described in L15. W18. Compare your measured results for output peak-to-peak signal amplitudes vout(p-p) and small-signal voltage gain av Compare to your theoretically predicted values from W17. W19. Comment on the shape of the output waveform – was it a clean sinusoid, or was there distortion? How did the distortion compare to the output of the circuit with degeneration resistor RE in part W11? EXCEEDING “SMALL SIGNAL” LINEAR RANGE W20. Plot a representative waveform showing distortion of the output triangle wave for large waveform amplitudes. Explain this behavior in light of the small-signal linear approximation. W21. Plot the output waveform, identifying the measured maximum and minimum voltage clipping levels. Identify the transistor operating regions corresponding to each limit. 11